Battery charger with automatic change from current to voltage mode control

ABSTRACT

A pair of silicon controlled rectifiers are alternately triggered into their forward conductive states at points relatively early in each alternating current half cycle to provide a maximum charging current when a substantially discharged battery is to be recharged. As the charging process progresses, the battery voltage increases and at a predetermined value of the battery voltage a smooth transition from the current-controlled mode to a voltage-controlled mode is inaugurated. After the change to the voltage-controlled mode has been completed, the SCRs are triggered into conduction much later in each cycle, thereby furnishing a very low or trickle current which can be continued indefinitely without damage to the battery. Simple adjustments are provided for determining the maximum current charging rate and the particular voltage at which the changeover is to begin. Provision is also made for preventing an initial sudden current surge when first turning on the power, thereby avoiding any unnecessary blowing of fuses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to battery chargers, and pertains moreparticularly to a charger which automatically reduces the chargingcurrent when a predetermined battery voltage is reached.

2. Description of the Prior Art

The prior art is replete with various types of battery chargers. Someoperate on only a time basis, switching from a rapid charge to a tricklecharge after a predetermined period of time has elapsed. Still othersprovide an uncontrolled maximum charging current initially without anyattempt to control the magnitude of the current during the rapidcharging period; when a predetermined battery voltage is reached, thenthere is a sudden reduction to the trickle charge. Various hardwaretechniques have been resorted to in the past in an effort to provide afast charge, yet not damage the battery. However, the prior artarrangements with which I am acquainted all have certain shortcomingswhich the present invention effectively overcomes.

SUMMARY OF THE INVENTION

One object of the present invention is to control constantly the maximumcharging current during the early charging period, and upon the batteryvoltage reaching a predetermined value to then effect a smoothtransition from the current-controlled mode to a voltage-controlled modewhich then continues until the battery is removed from the charger. Inthis regard, it is an aim of the invention to continue the initial highcharging rate for whatever period is practical, and then to provide atapering off of the charging current so that the eventual trickle chargeis only a fraction of the maximum rate, which can be continuedindefinitely without damage to the battery. Hence, it is within thepurview of the invention to minimize the overall time required forcharging a battery, yet never overcharge the battery so that it isdamaged.

Another object of the invention is to provide a charger that willfunction over a wide range of input voltages. More specifically, thevoltage can be from between 95 to 135 volts for a nominal 115 voltalternating current input and from 190 to 270 for a nominal 230 voltalternating current input. My charging system is, for all intents andpurposes, independent of input voltage.

Another object is to provide a battery charger that will function on arange of power frequencies. In this regard, public utility power systemsin the United States have standardized on a frequency of 60 Hz, whereasforeign countries in many instances employ 50 Hz systems. Batterychargers utilizing ferroresonant transformers must be operated on afrequency for which the transformer is designed. My battery chargerobviates the need for using a ferroresonant transformer and henceprovides a charging system that is for all intents and purposesindependent of frequency as well as input voltage.

Still another object of the invention is to prevent any sudden surge ofcharging current when the power is first turned on with a concomitantelimination of unnecessary fuse blowing.

Yet another object of the invention is to obviate the need for anymechanical switching devices, such as timers and relays, in going from ahigh charging rate to a low charging rate.

Briefly, my invention contemplates the controlled triggering of siliconcontrolled rectifiers so as to provide first a relatively constantcharging current for whatever period is needed and later to reduce thecurrent to a minimal or trickle value after the battery voltage hasreached a predetermined level. A current sensing circuit provides avoltage signal in accordance with the amount of charging current andexercises a dominant control during the high charging rate interval. Theterminal voltage of the battery undergoing charge is continually sensedbut remains ineffectual, or substantially so, until the desired batteryvoltage is reached. By means of a summing point, the resulting algebraicsum of the two voltages derived from the current and voltage sensing isapplied to one input of an error amplifier. The other input to the erroramplifier has a reference voltage impressed thereon which is set so thatwhen the battery voltage reaches a predetermined level, a changeoverfrom the current-controlled mode of operation to a voltage-controlledmode will be initiated. When there is a difference between the voltagessupplied to the error amplifier, then there is an output voltage orerror signal that is representative of the difference between the twoinput voltages to the error amplifier. The output voltage or errorsignal causes a transistor to conduct in accordance with the magnitudeof such output or error signal. Responsive to the state of conduction ofthe transistor is a timing capacitor which activates a triggering devicewhich in turn triggers the two silicon controlled rectifiers intoconduction, doing so at particular points in the various alternatingcurrent cycles. The successive triggering of each SCR during the earlyportion of a cycle causes a greater flow of current and a triggeringlate in the cycle produces a much lower charging current.

When the power is first turned on, a transistor functioning as a switchprevents the above-mentioned transistor from becoming conductive so thatno triggering action immediately occurs; however, as the above-mentionedtransistor becomes conductive, the triggering device turns on the SCRs,first late in the cycle and then progressively earlier in succeedingcycles until the required amount of maximum charging current is reached.This is quite rapidly (within a few cycles) but not instantaneously,thereby preventing the blowing of fuses unnecessarily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram exemplifying my battery charger;

FIG. 2 represents a typical AC voltage waveform to the charger;

FIG. 3 depicts a typical current waveform (before rectification), theshaded portions representing the portions during which the rectifiersare conducting to provide the high charging current rate;

FIG. 4 corresponds to FIG. 3, the small shaded portions illustrating thebrief conductive periods producing the trickle portion of the chargingprocedure;

FIG. 5 is a discharge profile of a typical battery, the dischargecurrent being plotted against time, and

FIG. 6 is a composite graph showing the relation between the batterycharging current and the battery voltage when employing my charger,typical current values being given at the left and typical voltagevalues being given at the right.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, my battery charger has been denoted generally bythe numeral 20. Included is a power supply constituting a transformer 21having a primary winding 22 provided with terminals 24 and 26 whichconnect to a conventional 50 or 60 Hz AC power source. The transformer21 is of conventional construction and not frequency dependent as areferroresonant transformers which are employed in many prior artchargers. While nominally 115 volts, the voltage can vary over a rangefrom 95-135 volts, it may be pointed out. The transformer 21, however,does employ a degree of magnetic decoupling between its primary coil 22and its secondary coil 28 to eliminate inordinately large peak currentsthat may otherwise flow in these coils 22, 28. The secondary coil 28 iscenter tapped at 34. Connected to the ends of the secondary winding 28are fuses 36 and 38, which, because of a precautionary feature later tobe described, need not be of the slow blowing type. Although referred tomore specifically hereinafter, two silicon controlled rectifiers 40 and42 will be merely mentioned at this time; these rectifiers change the ACcurrent supplied by the transformer 21 to DC current which is, ofcourse, essential when charging any battery. My battery charger 20additionally includes a pair of output terminals 54 and 55 to which thebattery 56 is connected during the charging thereof; more specifically,the positive side of the battery 56 is connected to the terminal 54 andits negative side to the terminal 55.

A current sensing resistor 70 is employed, which will hereinafter bemore specifically referred to. At this time, it will be mentioned thatthe ensuing description will be facilitated somewhat by assigningreference numerals to certain conductors (but not all of them) containedin the charger 20. In this regard, a conductor 78 connects the terminalat the center tap 34 to one end of the current sensing resistor 70, anadditional conductor 80 extending from the other end of the currentsensing resistor 70 to the output or load terminal 54. Still further,conductors 82 and 84 connect the cathodes of the silicon controlledrectifiers 40, 42 to a conductor 86 extending to the output or loadterminal 55.

To provide a voltage signal representative of the magnitude of chargingcurrent flowing to the battery 56, which must flow through the seriallyconnected current sensing resistor 70, a current sensing circuit 88 isemployed. The current sensing circuit 88 includes a PNP transistor 90having its base connected to the wiper arm 92 of a potentiometer 94, theconnection being through a resistor 96. Between the end of the resistor96 that is attached to the base of the transistor 90 and the end of thepotentiometer attached to the current sensing resistor 70 is a capacitor98. The emitter of the transistor 90 is connected to the same end of thepotentiometer 94 as the capacitor 98 is connected, being connectedthrough an emitter resistor 100. The collector of the transistor 90 isconnected to one end of a resistor 102, the other end of the resistor102 being attached to the anode of a diode 104.

The foregoing completes the description of the current sensing circuit88. However, it will be perceived that the cathode of the diode 104 isconnected to a summing point labeled 106. The adjustment or setting ofthe potentiometer 94, more specifically its wiper arm 92, causes thetransistor 90 to become responsive to current flowing through thesensing resistor 70, its conduction causing current to flow through theresistor 102, the diode 104 to the summing point 106.

The summing point 106 is connected through a resistor 108 to a voltagedivider composed of a pair of resistors 110, 112. The junction 114between the resistors 110, 112 provides a voltage that is representativeof the battery voltage existing between the terminals 54, 55, thisvoltage being applied to the summing point 106 through the resistor 108.Thus, the point 106 algebraically sums the voltage forwarded from thecurrent sensing circuit 88, which is representative of the value ofcharging current, and the voltage supplied by the voltage divider 110,112, which is representative of the battery voltage as it changes duringcharging. The resistor 108 and a capacitor 116 perform an averagingfunction for the resulting potential or voltage at the summing point106, the capacitor 116 being connected between the summing point 106 andthe conductor 86.

At this time, attention is directed to a voltage regulator 118, such asthe linear integrated circuit chip, type 723, part 723DM, this being a14-lead DIP package, manufactured by Fairchild Semiconductor, 464 EllisSt., Mountain View, Calif. 94040. Whereas the regulator chip 118 isprovided with 14 leads, only eight of these leads are actually used. Itwill help to label the various leads with the same numbers used in thedescriptive literature published by Fairchild, the numerals 4-7 and10-13 appearing within the rectangle denoting the chip regulator 118. Itis not believed necessary to set forth herein the equivalent circuit forthe regulator 118; however, it will be helpful to depict the erroramplifier 120 within the integrated chip that has its two inputterminals connected to the leads 4 and 5 and its output terminalconnected to the lead 10. Any difference between the voltages applied tothe leads 4 and 5 is reflected as an error voltage or difference signalat the output lead 10. Although not essential for an understanding ofthe present invention, an integrator for the regulator 118 comprises acapacitor 122 and resistors 124, 126. One end of the resistor 126 isconnected to the summing point 106 and also to the capacitor 116,whereas the other end of the resistor 126 is connected to the input lead4 of the regulator 118, thereby applying the algebraically summedvoltage at the point 106 to one input of the error amplifier 120. Areference voltage is applied to the other input of the amplifier 120 vialead 5, this being supplied by a voltage divider composed of resistors128, 130 plus a potentiometer 132 connected between the resistors 128and 130. The wiper arm 134 of the potentiometer 132 connects directly tothe input lead 5 of the regulator 118. In this way, the referencevoltage can be varied, and, as will become clear hereinafter, thesetting of the wiper arm 134 determines the voltage at which the circuit44 switches from its current mode to its voltage mode.

It will be seen that the lead 7 of the regulator 118 is connecteddirectly to the conductor 86. The leads 11 and 12 are connected to thecathode of a Zener diode 136, the anode of the Zener diode 136 beingconnected to the conductor 86. The Zener diode 136 merely limits thevoltage that is supplied to the leads 11 and 12 of the regulator 118. Itmight be mentioned at this stage that the leads 11, 12 are alsoconnected to a resistor 138 that is in circuit with a diode 140, theanode of the diode 140 being connected to the other end of thepotentiometer 94 of the current sensing circuit 88 and also to thecollector of another transistor 143 which functions as a switchingdevice to prevent initial charging current surging as will hereinafterbe explained.

As earlier mentioned, the output lead 10 carries a voltage that isindicative of the difference or error between the voltages applied tothe input leads 4, 5. The lead 10 is connected to one end of a voltagedivider comprised of resistors 142 and 144. It is the junction 145between the resistors 142, 144 that is connected to the emitter of a PNPtransistor 146 having its base connected to the conductor 86. Thecollector of the transistor 146 is connected directly to one side of atriggering device 148. The other side of the triggering device 148 isconnected to the anodes of a pair of diodes 150 and 152, the cathodes ofthese diodes being connected to the gates of the previously mentionedsilicon controlled rectifiers 40 and 42. Also, the cathodes of thediodes 150 and 152 are connected to one end of resistors 154 and 156,the other ends of the resistors 154 and 156 being connected to theanodes of the SCRs 40 and 42, respectively.

For the purpose of determining the time at which the triggering device148 conducts, a capacitor 158 is connected between the side of thetriggering device 148 which is attached to the collector of thetransistor 146 and a resistor 160, the resistor in turn being connectedto the conductor 86. The same side or plate of the capacitor 158 that isconnected to the resistor 160 is also connected to the anodes of twodiodes 162 and 164, the cathodes of these diodes 162 and 164 beingattached to the fuses 36 and 38, respectively (and also the cathodes ofthe SCRs 40, 42).

The transistor 142 that has been previously mentioned has its baseattached to one end of a resistor 166, the other end of the resistor 166being connected to the anodes of diodes 168, 170 and to one side of acapacitor 172. The cathode of the diode 168 is connected to the fuse 36(and the cathode of the rectifier 40), whereas the other side of thecapacitor 172 is attached to the conductor 86.

OPERATION

Having presented the foregoing, the operation of the charger 20exemplifying my invention is as follows. When the input terminals 24 and26 of the primary winding 22 of the transformer 21 are connected to anappropriate AC power source, then the secondary winding 28 is energizedso as to cause a voltage, such as that exemplified by the waveform 174of FIG. 2, to be supplied to the control circuit portion of my charger20 which is connected to the opposite ends of the secondary winding 28via the fuses 36, 38 and also to the center tap labeled 34. As thevoltage at the center tap 34 increases in a positive direction, there isa flow of current via the conductor 78, through the current sensingresistor 70, thence over the conductor 80 to the output or load terminal54, then through the positive terminal or side of the battery 56undergoing charge, from the negative side of the battery 56 through theoutput terminal 55, then via the conductor 86, which can be considered"ground", to the junction of the conductors 82 and 84. If the end of thesecondary winding 28 of the transformer 21 connected to the fuse 36 isnegative at the particular moment, then there is a flow of currentthrough the conductor 82 to the anode of the SCR 40.

However, for the current to flow through SCR 40, its gate must have anappropriate triggering voltage applied thereto through the diode 150.This can only be accomplished if the triggering device 148 isconductive. The conduction of the triggering device 148 depends upon thecharge on the timing capacitor 158. However, the charge on thiscapacitor 158 is influenced by the amount transistor 146 conducts. Theconductive state of transistor 146 in turn depends upon the outputvoltage from the lead 10 of the regulator 118. The error amplifier 120contained in the regulator 118 produces an error voltage depending uponthe difference between the voltage applied to the lead 4 and thatimpressed on the lead 5. It will be understood that the particularvoltage appearing at the summing point 106 determines the magnitude ofthe output signal from the lead 10, being compared with the referencevoltage supplied by the potentiometer 132. When the error or differenceis reduced to zero, there will be an output from the amplifier 120 viathe lead 10 to transistor 146 which causes properly timed trigger pulsesto be applied to SCR 40 and SCR 42 which, in turn, cause sufficientcurrent to flow to the battery 56 to maintain a zero error signal at theamplifier 120.

As far as the potential appearing at the summing point 106 is concerned,it is derived from the current sensing circuit 88 during the currentmode operation; it is derived from the divider network comprised of theresistors 112, 114 during the subsequent voltage mode operation,however. Regarding the current mode operation, the amount of currentflowing through the sensing resistor 70 determines the voltage suppliedto the potentiometer 94. The potentiometer 94 is in parallel with thecurrent sensing resistor 70. More specifically, an electrical path canbe traced from the center tap 34, the conductor 78, through thepotentiometer 94, through the collector and emitter of the transistor143 to the conductor 80. It will be recognized that the conductor 80 hasone end attached to the current sensing resistor 70, thereby completingthe parallel path just traced. The transistor 143 is normally saturated,but performs a switching function as will presently be clarified.

At this time, it will be recognized that the magnitude of the voltagedrop across the potentiometer 94 influences the bias voltage applied tothe base of the transistor 90 through the wiper arm 92 of thepotentiometer 94 and through the resistor 96. Also, the capacitor 98 inconjunction with the resistor 96 averages the voltage, and hence anaverage of the charging current, which voltage is impressed on the baseof the transistor 90. Being a PNP transistor, when the bias applied tothe base thereof goes sufficiently negative, the transistor 90 becomesconductive so as to cause current to flow through the resistor 102 andthe diode 104 to the summing point 106 and thus applies a potentialthereto representative of the charging current flowing to the battery56. Since the summing point 106 is connected to the junction 114 of thevoltage divider composed of the resistors 110, 112, the particularvoltage appearing at the summing point 106 is an algebraic sum of thevoltage supplied by the current sensing circuit 88 and the voltagesupplied by the voltage divider 110, 112. It should be noted that thevoltage at the summing point 106 cannot be less than the voltage at thejunction 114 because of the action of the diode 104.

It is the current supplied by the transistor 90 and the current suppliedthrough the resistor 108 that is responsible for charging the capacitor116, doing so, of course, via the summing point 106. More specifically,when the capacitor 116 is charged to about, say, several millivolts morethan 3 volts, and with a reference voltage of, say, 3 volts, then itfollows that the few millivolt difference applied to the inputs of theerror amplifier 120 via the leads 4, 5 of the regulator 118 will causethe output voltage supplied from the lead 10 to assume a valuerepresentative of the difference between the amplifier's inputs. Morespecifically, the regulator 118, under the several millivolt error asdescribed above, will cause the transistor 146 to conduct only slightly,thereby delaying the trigger pulses to SCR 40 and SCR 42 andconcomitantly causing these SCRs to conduct relatively late in each halfcycle, as should be evident from the shaded portions of the waveform 176in FIG. 4. Conversely, with a reference voltage of, say, 3 volts and avoltage of a few millivolts across the capacitor 116 less than thereference voltage, the regulator 118 will cause the transistor 146 toconduct heavily, thereby causing triggering of the SCRs 40, 42 earlierin each half cycle, as illustrated by the waveform 175 of FIG. 3, which,of course, results in an increased current flow through the battery 56.It will be recognized that the change in current flow to the battery 56caused by the regulator 118 is in a direction to cause the absolutevoltage difference between the leads 4, 5 of the regulator 118 todecrease. Since the transistor 146 is a PNP transistor, the flow ofcurrent through resistor 142 (and resistor 144) will cause the emitterto become more positive with respect to the conductor 86 (ground) thanthe base of the transistor 146; stated somewhat differently, the basebecomes more negative with respect to the emitter and to produce aconduction of the transistor 146 in accordance with the output voltagesignal, the amount of conduction determining the charging rate for thecapacitor 158 and hence the triggering speed or rate of the device 148.

It should be understood that as the charging of the battery 56 proceeds,the voltage between the output or load terminals 54 and 55 willincrease, the increase reflected at the junction 114 of the voltagedivider 110, 112 causing the summed voltage at the point 106 to bemaintained at the reference voltage applied to lead 5 of the regulator118 to an increasing degree by the voltage of junction 114, and to adecreasing degree by the current through diode 104. It will beappreciated that during normal operation of the charging system thevoltage difference between the leads 4, 5 of the regulator 118 is alwayswithin a few millivolts of zero due to the aforesaid regulating actionof the regulator 18. Thus, the transition from a current mode control toa voltage mode control, as would be the case for an increasing voltageacross the output terminals 54 and 55, is accomplished by maintainingthe voltage across the capacitor 116 with a current through resistor 108rather than by means of a current through the diode 104. When thevoltage at the summing point 106 equals the reference voltage suppliedby the potentiometer 132, then there is a zero difference applied to theinput leads 4, 5 of the regulator 118 causing an output or controlvoltage having a value representative of this zero difference to appearon the lead 10.

Consequently, when a charging period is initiated, for example, with anominally discharged battery 56 and with an input voltage (see waveform174 in FIG. 2) to the primary winding 22 of the transformer 21 at thelow end of the specified 95-135 volt range (or the low end of a 190-270volt range, as the case may be), there will be a relatively large outputvoltage on the lead 10 of the regulator 118, and the regulator 118,under these operating conditions, compensates for the relatively lowinput voltage and the relatively high current demand of the battery 56.This larger output signal via lead 10 of the regulator 118 causes thetransistor 146 to conduct heavily with the consequence that thecapacitor 158 is rapidly charged. As soon as the capacitor 158 ischarged to the proper triggering voltage (on the order of 8 volts), thedevice 148 conducts so as to forward a gating signal to the SCR 40during one-half of the cycle and to SCR 42 during the other half. Inother words, as the input voltage 174 (FIG. 2) supplied to thetransformer 21 goes positive, there will be a turning on of the SCR 40earlier in the positive going half of the cycle, the same thing holdingtrue for the SCR 42 on the negative part of the cycle. The waveform 175pictured in FIG. 3 in intended to visually portray what happens, theshaded areas indicating a greater current flow. Thus, a phase controlstarts earlier when the triggering device 148 acts during the earlyportion of the cycle, which it does when the transistor 146 isconducting more heavily than when its state of conduction is reduced.Resistor 160 supplies a discharge path from the capacitor 158 to theconductor 86 (ground), the discharge path also including the base of thetransistor 146 which base is connected directly to the conductor 86.Thus, the capacitor 158 does not retain its charge but continuallycharges and discharges, the rate changing in accordance with the amountof conduction of the transistor 146. It will be understood that thecapacitor 158 is always discharged to the same voltage level at the endof each half-cycle by forward conduction through the collector-base pathwithin the transistor 146, thereby providing the necessary triggeringstability.

In actual practice, when the battery voltage is less than 28.8 volts,then the current sensing circuit 88 causes the battery charger 20 tooperate in its current-controlled mode. As the voltage increases due tothe battery 56 becoming more fully charged, the 28.8 volts thresholdvalue is reached which causes the circuit to change into itsvoltage-controlled mode, thus causing only a small amount or tricklecurrent to be supplied via the output terminals 54, 55 to the battery56.

It may be helpful to refer to FIGS. 5 and 6 at this time which showdischarging and charging profiles, respectively, for a given battery. InFIG. 5 the current curve has been assigned the reference numeral 180,the ordinate representing amperes and the abscissa hours. In FIG. 6 bothvoltage and current curves 182 and 184 have been plotted against time,the ordinate at the left representing amperes and the ordinate at theleft volts. When the battery voltage reaches 28.8 volts, then, as can bediscerned from FIG. 6, there is a relatively rapid decrease in theamount of current, the decreased current then continuing indefinitelywithout battery damage until the battery 56 is disconnected from theterminals 54, 55.

It will be recalled that it was earlier stated that the potentiometer 94is in parallel with the current sensing resistor 70. More specifically,the potentiometer 94 and the collector-emitter circuit of the transistor143 are connected in parallel with the current sensing resistor 70.Obviously, the transistor 143 must be conducting in order for thecurrent sensing circuit 88 to exercise its control function. To achievethis, the transistor 143 is normally saturated whenever the circuit isoperating. Not only is there then a flow of current through thepotentiometer 94 and the collector-emitter circuit of the transistor143, but there is also a flow of current through its emitter and base,through the resistor 166, the diode 168 to the fuse 36, if that end ofthe transformer winding 28 is negative with respect to its center tap34, but through the diode 170 to the fuse 38 if the other end of thetransformer winding 28 is negative with respect to the center tap 34.The capacitor 172 will have a zero charge that causes the base currentto flow as just explained.

However, when the terminals 24 and 26 are disconnected from the AC powersource, which has not been shown, then there is, of course, no powersupplied to the circuit from the transformer 21 and therefore, quiteobviously, no current flows through the diodes 168, 170. This shuts offthe transistor 143. When current is flowing through the transistor 90,though, as it does during the charging of the battery 56, this causes acharge to be placed on the capacitor 116 which is utilized in the normaloperation of the circuitry comprising the charger 20, the charge of thecapacitor 116 applying a voltage obtained from the summing point 106 tothe input lead 4 of the regulator 118. The charge on the capacitor 116is increased when the power is disconnected because the transistor 143ceases to conduct, causing capacitor 98 to retain (or increase) itscharge, which, in turn, causes transistor 90 to conduct. In this regard,it will be observed that the emitter of transistor 90 is connected tothe positive side of the battery 56 through resistors 100, 70, conductor80 and output terminal 54; the collector of this transistor is connectedto the negative side of the battery 56 through resistor 102, diode 104,resistors 108, 112 and output terminal 55. And, as in normal operation,the conduction of transistor 90 causes capacitor 116 to retain (orincrease) its charge with capacitor 116 applying, say a 5 volt potentialto lead 4 of regulator 118, and with lead 5 of the regulator 118 havinga 3 volt potential applied thereto, then the output of regulator 118 atlead 10 will have nearly zero potential, resulting in no conduction oftransistor 146. With the transistor 146 non-conductive, then thetriggering device 148 does not fire, for there is no voltage deliveredto the timing capacitor 158 and it must be charged to the necessarydegree before the triggering device conducts.

After turning the power back on, then the transistor 143 again saturateswhich turns off transistor 90, the capacitor 98 then discharging. Thisstops the flow of current that was previously holding capacitor 116 at apotential greater than that of lead 5 of the regulator 118, but notinstantaneously. Until this occurs, though, the capacitor 116 dischargesslowly through resistor 108 and resistor 112. As soon as the chargeremaining on the capacitor 116 results in a voltage equal to thereference voltage supplied by the potentiometer 132, then there will bean increasing output voltage on the lead 10 of the regulator 118, andthe transistor 146 conducts by reason of the voltage supplied by theoutput lead 10.

The above results in the timing capacitor 158 becoming charged and whensufficiently charged, then the triggering device 148 is caused toconduct with the consequence that the firing action for the siliconcontrolled rectifiers 40 and 42 is resumed. This does not re-occurinstantaneously, though, for there are several cycles in which thefiring of the rectifiers 40, 42 is later in the cycle. Stated somewhatdifferently, the firing is delayed until toward the end of each halfcycle (see waveform 176 in FIG. 4) and this delay is steadily decreasedby virtue of the progressively more rapid charging of the timingcapacitor 158 until the triggering action provided by the device 148takes place quite early in each cycle (see waveform 175 in FIG. 3) whichwould supply the full amount of charging current to the battery 56. Thisslow "turn on" (even though only for a few cycles) is quiteadvantageous, because the fuses 36 and 38 are prevented fromunnecessarily blowing.

I claim:
 1. A battery charger for connection to a source of AC powercomprising:a rectifier means for supplying charging current to a batteryto be charged; a resistor in series with said rectifier means forproviding a voltage drop thereacross caused by the flow of chargingcurrent through said resistor; a first potentiometer connected inparallel with said resistor and having an adjustable contact; a firsttransistor having a base connected to said adjustable contact so as tovary the conduction of said transistor in relation to the voltage dropacross said resistor, said transistor providing an output voltage signalrepresentative of the charging current; a voltage divider in parallelwith said battery providing a voltage signal representative of saidbattery voltage; summing point means for algebraically summing saidoutput voltage signal of said transistor and said voltage representativeof said battery voltage; an error amplifier having a pair of inputs andan output, one of said inputs connected to said summing point and theother of said inputs connected to a reference voltage, said amplifierproducing an output signal representative of the difference between saidvoltages applied to said inputs; means for controlling said rectifiermeans in accordance with said amplifier output signal to increase theflow of charging current when said battery voltage is below a thresholdvalue and to decrease the flow of charging current when said batteryvoltage reaches said threshold value; a second potentiometer having anadjustable contact connected to said other of said inputs of said erroramplifier to provide a variable reference voltage; and means forchanging the voltage applied to said one input of said error amplifierin a direction generating an error amplifier output signal controllingsaid rectifier means to reduce the charging current when the charger isinitially connected to a source of AC power.
 2. A battery charger inaccordance with claim 1, wherein said means for initially changing thevoltage applied to said one input of said error amplifier furthercomprises:a second transistor for increasing the conduction of saidfirst transistor thereby temporarily increasing the voltage normallyrepresentative of the amount of said charging current so that saidsumming point applies a voltage to said one input of said erroramplifier sufficiently greater than said reference voltage so as toreduce the resulting signal at the said output of said error amplifierto zero.
 3. A battery charger comprising:a pair of input terminals forconnection to a source of AC power; a transformer including a primarywinding connected to said input terminals and a secondary winding havinga center tap; a pair of output terminals for connection to a battery tobe charged; a first rectifier having an anode, cathode and controlelectrode, said anode and said cathode of said first rectifier being inseries with one end of said secondary winding and one of said outputterminals; a second rectifier having an anode, cathode and controlelectrode, said anode and said cathode of said second rectifier being inseries with the other end of said secondary winding and said one outputterminal; a current sensing resistor connected in series between thecenter tap of said secondary winding and the other of said outputterminals; means responsive to the potential drop across said currentsensing resistor to provide a first voltage signal representative of theamount of charging current flowing to said other of said outputterminals, said means including a potentiometer having a fixedresistance connected in parallel with said current sensing resistor anda wiper arm movable with respect to said fixed resistance, a capacitorin parallel with said wiper arm, a first transistor having a collector,emitter and base, said base being connected to said wiper arm and to oneside of said capacitor, said emitter being connected to the other sideof said capacitor; a voltage divider for providing a second voltagesignal representative of the voltage of said battery as it is beingcharged; means for summing said first and second voltage signals toprovide an algebraically summed signal, said collector of said firsttransistor being connected to said summing means; means for comparingsaid summed signal with a reference voltage signal to provide a controlvoltage signal indicative of the difference between said summed voltagesignal and said reference signal; means connected to said controlelectrode for increasing the charging current flow when said batteryvoltage is below a threshold value and decreasing said charging currentwhen said battery voltage reaches a threshold value, said meansincluding a triggering device and a timing capacitor, the rate at whichsaid capacitor is charged depending on the value of said control voltagesignal, whereby said triggering device becomes conductive more rapidlywhen said control voltage signal is relatively great to cause conductionof said rectifiers earlier in each AC half cycle than when said controlvoltage signal is of a lesser value; a second potentiometer foradjusting the value of said reference voltage to change the time atwhich said rectifiers are triggered when said voltage of said batteryhas reached a desired level; and a second transistor having a collector,emitter and base, said collector in said emitter being in series withsaid first potentiometer, and said base being connected to the cathodesof said rectifiers whereby said second transistor is renderednon-conductive when said input terminals are first connected to said ACpower source thereby increasing the value of said first voltage signalto delay the triggering of said rectifiers until relatively late inseveral half cycles of said AC power source.